LP Gas On-Demand Water Heater

[OkTx]

SO I finally got my off-grid 48v 4500w system installed and connected. Couldn't be happier. So nice to have power without hearing a generator running. I had planned on installing an LP gas, tankless, on-demand water heater. When I started looking closely at them I began noticing they consume a lot of power. Some say up to 40kW. That seems like a LOT of power to heat water with propane. Am I reading the specs right?

My water will be supplied from an on-site well so it may be a bit cool 60'ish F I'd guess. What are you folks using for whole house water heaters? I'm just heating water for a 2 BR / 2 BA cabin that will normally only have 2 people in it. I'd venture to say that only the kitchen sink and maybe the washer would run at the same time. I can't see both of them running simultaneously for more than a minute or two.

 

[John Frum]

Some say up to 40kW.

40 kw should heat the water without propane.
And electric water heater with tank is only 30amps @ 240 VAC with is 7.2kw.

 

[John Frum]

this electric tankless unit is 36 kw https://www.homedepot.ca/product/st...gpm-electric-tankless-water-heater/1001241704

 

[Picasso]

I love and use https://www.eccotemp.com/ can buy all parts for them online.

 

[Steve_S]

I am OFFGRID, use Two On-Demand Heaters. The first one is a Takagi TH3M which is used exclusively for the Radiant Heating (it has propylene glycol in it). The second is an EccoTemp FVI12. Both are Propane fuelled. They use SQUAT for power ! The flow sensing is the only power used while in standby and it's like 0.01A when running only the electronic ignition is use to light them. With the Radiant System the pump draws some (it's an efficient Grundfos) but total about 1.2A when that whole system runs. On the Hot Water, again the Heater itself uses little just as noted above. When the Water pump kicks on (Soft Start Grundfros SQ5, 260' deep) it starts at 500W and stages to 1100W when max pressure is reached.

The Takagi is an awesome heater and we are talking a Tier-1 product here so the price reflects that, they re selling around $1,000 USD
The EccoTemp on the other hand is a cheap Chinese Heater, it works pretty well BUT is persnickety with the settings, luckily I installed it in the bathroom, The FVI12 was cheap when I bought it, around $350 or so. I Would suggest that IF your looking at EccoTemp then look at the FVI20 or better and up series. IF Budget allows for it, go with Takagi !!!

Whoever told you that the LPG On-Demands use that much power was sniffing Gas Fumes & puffing BAD Doobies or something while drinking homemade Radiator Hooch! Only the REALLY Big Electric ones could / would use that much power and that would be with a bad installation (as in poorly planned & implemented).

- The REAL KEY to your hot water management is to have the On-Demand heater as close to the point of use as possible.
- Provide 3/4 or 7/8 water pipe to it and THEN split the cold and to hot water heater to 1/2" plumbing (as specd)
- IF you have a Cold Well and need to raise the temp a lot, you can use a PRE-MIX (not a post mix) Valve to reinject a "little" hot water back into the incoming cold water stream to raise the incoming temp and reduce the heat demand on the appliance and save fuel.
- NOTE: Some On-Demand heaters are finicky with water saver / flow restricted taps, aerators and such, as they keep the pressure too low for the flow sensor to read & start the heater. Most have resolved that but beware of it and look for that in the specs, I forget the number you need to look for at this time... been a while.
- Use a proper Installation Kit (generally NOT provided with the heater, unless you get a "complete kit") so that you have the appropriate valving and controls for hookup & the maintenance. Rheem Tankless Service Valves like these.

Hope that helps, Good Luck
Steve

 

[Jackfre]

I represented Rinnai for 27 yrs. you are miss reading the specs. Stand-by power is 2 w, operation is 40 w, and if the freeze protection kicks on it is 80-90 w. Properly placed/installed there will be no need for freeze protection. Don’t over-buy. What are your loads? You can buy a 120, 150, 180 or 199kbtu unit. As previously noted, placement is key for quick HW delivery. Put it out in the N 40 and you will wait. Pipe size matters here too. Look at the cross sectional area of 1/2, 3/4 & 1” pipe and it is .19, .44 and .77 sq“. that equates to vol. Today, I would only buy the Rinnai Sensei. There are many tankless units that are quite excellent. The reason I say Sensei is that it was designed for rapid service/repair when it becomes necessary. I can Have the heat exchanger on the floor in 10 minutes. It is unlikely that would be necessary but in years to come you can still protect the investment. Rinnai’s are outstanding

 

[TKB4]

I represented Rinnai for 27 yrs. you are miss reading the specs. Stand-by power is 2 w, operation is 40 w, and if the freeze protection kicks on it is 80-90 w. Properly placed/installed there will be no need for freeze protection. Don’t over-buy. What are your loads? You can buy a 120, 150, 180 or 199kbtu unit. As previously noted, placement is key for quick HW delivery. Put it out in the N 40 and you will wait. Pipe size matters here too. Look at the cross sectional area of 1/2, 3/4 & 1” pipe and it is .19, .44 and .77 sq“. that equates to vol. Today, I would only buy the Rinnai Sensei. There are many tankless units that are quite excellent. The reason I say Sensei is that it was designed for rapid service/repair when it becomes necessary. I can Have the heat exchanger on the floor in 10 minutes. It is unlikely that would be necessary but in years to come you can still protect the investment. Rinnai’s are outstanding

I realize this is an old thread . I have a rinnai 98 Natural Gas unit that I DIY installed almost 3 years ago and have zero problems from. I first started using electric tankless at office about 12 years ago and zero problems with it too(not a rinnai) . I do think many people do not realize the size gas supply line they will need for the larger units and also that they will need 120v for igniter and fan . I would like to hear what you think about providing the unit with preheated water from solar pv heated tank or heat exchanger. I am thinking about changing the orifice to use it with LP gas also.

 

[Jackfre]

You are correct on the gas line size issue. Amongst all tankless manufacturers it is the number one problem. Yes, also, you do need 120v, but look at the actual wattage consumed. The Rinnai’s I worked with drew about 40 W when they were at max fire. Standby is only 2W. If the unit needed freeze protection then you are in the 80-100w range. Install it so that is not an issue. As I see it the issue with preheating water for a gas tankless is on the margins, meaning, when the solar water is in the lower temp ranges. Some manuf say just feed the solar water directly to the tankless. Maybe it works, but it cycles the hi-limits like crazy and I don’t like to superfluously drive a safety device. So, what to do? Given that you need a tempering valve at the supply to the home, the issue is how to control the flow. An aquastat at the outlet of the tank sensing hot water temp controlling a 3-way valve that sends HW to the TV, by-passing the tankless, when it is adequate temp to supply to the house. When the tank cools off sufficiently the A-stat will divert water to the tankless to bring it along. Bonomi makes an electrical 3-way with very low turning torque, meaning low power to drive it, and they are excellent products. I can’t say others are any better or worse, but do your homework. The other issue is, and this requires a bit of patience, at what temp do you divert the water to the tankless. The reason this is an issue is that you have to look at the specs of the tankless. They have a min water flow and gas input to initiate fire. For instance, My Rinnai non-condensing RL-75 requires .4 gpm and the min fire of 10 kbtu. If you move to the condensing 95% units you have .4gpm, but 15kbtu min input. Run the numbers. GPM X Delta T X 500=BTU input. I did lot of solar thermal back in the late 70’s/early 80’s and there are those issues with margin temps.

 

[Hedges]

SO I finally got my off-grid 48v 4500w system installed and connected. Couldn't be happier. So nice to have power without hearing a generator running.

If you have an off-grid PV system, you either have too much power or too little.
Do the batteries ever get fully charged? If so, that could enable a tank-type water heater to use surplus power to heat water.
That could be your only water heater, or a pre-heater before your propane one.

 

[TKB4]

You are correct on the gas line size issue. Amongst all tankless manufacturers it is the number one problem. Yes, also, you do need 120v, but look at the actual wattage consumed. The Rinnai’s I worked with drew about 40 W when they were at max fire. Standby is only 2W. If the unit needed freeze protection then you are in the 80-100w range. Install it so that is not an issue. As I see it the issue with preheating water for a gas tankless is on the margins, meaning, when the solar water is in the lower temp ranges. Some manuf say just feed the solar water directly to the tankless. Maybe it works, but it cycles the hi-limits like crazy and I don’t like to superfluously drive a safety device. So, what to do? Given that you need a tempering valve at the supply to the home, the issue is how to control the flow. An aquastat at the outlet of the tank sensing hot water temp controlling a 3-way valve that sends HW to the TV, by-passing the tankless, when it is adequate temp to supply to the house. When the tank cools off sufficiently the A-stat will divert water to the tankless to bring it along. Bonomi makes an electrical 3-way with very low turning torque, meaning low power to drive it, and they are excellent products. I can’t say others are any better or worse, but do your homework. The other issue is, and this requires a bit of patience, at what temp do you divert the water to the tankless. The reason this is an issue is that you have to look at the specs of the tankless. They have a min water flow and gas input to initiate fire. For instance, My Rinnai non-condensing RL-75 requires .4 gpm and the min fire of 10 kbtu. If you move to the condensing 95% units you have .4gpm, but 15kbtu min input. Run the numbers. GPM X Delta T X 500=BTU input. I did lot of solar thermal back in the late 70’s/early 80’s and there are those issues with margin temps.

Thanks for reply. It sounds like if I keep thermostat on Pre heated water turned down low enough that I might not need the diverting type valve say 100 degrees ? I am not wanting to get that complicated for now but maybe in the near future. I am really wanting to try a heat exchanger type pre heater with a large capacity like 200 or 300 gallons . Now I am also considering a recirculation pump to provide more "instant" at least warm water to the farthest runs. My house is large and has runs of up to 100 ft.
I wish the builder had designed better but they did initially have 2 50 gallon water heaters so when I made it one systems some runs are 20 ft or so longer than they were. Have you seen the guy that has videos about design to reduce water use (hot and cold) ? He uses a main trunk line then a lot of 1/4 inch lines as terminations and advocates for codes etc to change also. I can't remember his name . The videos are several years old and I dont think they are on You tube.

 

[TKB4]

Thanks for reply. It sounds like if I keep thermostat on Pre heated water turned down low enough that I might not need the diverting type valve say 100 degrees ? I am not wanting to get that complicated for now but maybe in the near future. I am really wanting to try a heat exchanger type pre heater with a large capacity like 200 or 300 gallons . Now I am also considering a recirculation pump to provide more "instant" at least warm water to the farthest runs. My house is large and has runs of up to 100 ft.
I wish the builder had designed better but they did initially have 2 50 gallon water heaters so when I made it one systems some runs are 20 ft or so longer than they were. Have you seen the guy that has videos about design to reduce water use (hot and cold) ? He uses a main trunk line then a lot of 1/4 inch lines as terminations and advocates for codes etc to change also. I can't remember his name . The videos are several years old and I dont think they are on You tube.

I also think I saw where some gas tankless generate their own power with the gas and dont require electrical hookup but not necessary for me and yes I was aware of the low power standby about 2 watts. There is or was a gas pack or gas furnace that also generated its own power even for fan so that it would function with grid down without connection to electricity. Both interesting but I would wonder if there is higher failure rate because of that feature.

 

[Hedges]

Now I am also considering a recirculation pump to provide more "instant" at least warm water to the farthest runs. My house is large and has runs of up to 100 ft.

Requires a return line for from the far end, of course.

The pipe looses heat, not as well insulated as water heater. So continuously recirculating water to keep warm water in the pipe means continuously consuming energy.

Perhaps a point-of-use water heater at the far end would be an alternative. A small tank-type water heater, fed by the cooled off water in the pipe followed by hot water from the water heater.

 

[TKB4]

Requires a return line for from the far end, of course.

The pipe looses heat, not as well insulated as water heater. So continuously recirculating water to keep warm water in the pipe means continuously consuming energy.

Perhaps a point-of-use water heater at the far end would be an alternative. A small tank-type water heater, fed by the cooled off water in the pipe followed by hot water from the water heater.

I am aware of energy loss but it would be the "free" energy except for the recirculation motor which would also be free. I am not considering point of use water heaters for various reasons.

 

[Jackfre]

If you go with recirc you have to map out your HW system to make the recirc system function properly. With a large floorpan as you describe, you should consider re-locating the water heater in the best location for minimizing water waste and what always seems like an interminable wait, especially at the kitchen sink. Running a dedicated 3/4" pex return line is your best bet. Then you need to consider how you control the recirc system. You can run it on a timer or an aqua stat, but what you really need is an on-demand recirc control. I installed Rinnai's wireless recirc control at my daughters home. Whomever piped that house should be horsewhipped. I ran the re-circ line rom the kitchen to the water heater. We installed a wireless motion sensor up in the corner of the ceiling in the M Bath. entering the bath would make the circulator and fire the unit. In the kitchen I installed the low profile wireless push button up under the lip of the cabinet next to the sink. Push the button and within about 10-12 sec you had HW. Without the recirc you waited almost 3 minutes. Unbearable. The kids are very happy with it. I have also solved delay problems by running a dedicated pipe from the WH to the kitchen. I would use 3/8" tubing. When you look at cross-sectional area of pipe you have 1/2" at .19 cu', 3/4@ .44 and 1" at .77. That equates to volume you have to move. 100' of 3/4 equals about 2.5 gallons. Push the numbers around a bit and do actual flow tests at the individual faucets. It will be shocking.

 

[Hedges]

Or for a budget-friendly solution, just drill out the restrictors of those 2 gpm faucets.
Even if you have to buy water, when the price is $0.01/gallon, running the tap is cheaper than those silly recirculation systems.
If you feel bad about it, catch the water in a bucket and use it to water plants or flush the toilet.

I attempted to explain cost/risk etc. to my 98 year old mother who insisted on carrying a container of recycled rinse water up the stairs (therefore not holding the handrail).
"One gallon of water costs $0.01, the last time you fell and hit your head the bill was $50,000; just use the faucet and buy the water. You have enough money."
But no, her skull is too thick. She insists on keeping her old habits.

 

[TKB4]

If you go with recirc you have to map out your HW system to make the recirc system function properly. With a large floorpan as you describe, you should consider re-locating the water heater in the best location for minimizing water waste and what always seems like an interminable wait, especially at the kitchen sink. Running a dedicated 3/4" pex return line is your best bet. Then you need to consider how you control the recirc system. You can run it on a timer or an aqua stat, but what you really need is an on-demand recirc control. I installed Rinnai's wireless recirc control at my daughters home. Whomever piped that house should be horsewhipped. I ran the re-circ line rom the kitchen to the water heater. We installed a wireless motion sensor up in the corner of the ceiling in the M Bath. entering the bath would make the circulator and fire the unit. In the kitchen I installed the low profile wireless push button up under the lip of the cabinet next to the sink. Push the button and within about 10-12 sec you had HW. Without the recirc you waited almost 3 minutes. Unbearable. The kids are very happy with it. I have also solved delay problems by running a dedicated pipe from the WH to the kitchen. I would use 3/8" tubing. When you look at cross-sectional area of pipe you have 1/2" at .19 cu', 3/4@ .44 and 1" at .77. That equates to volume you have to move. 100' of 3/4 equals about 2.5 gallons. Push the numbers around a bit and do actual flow tests at the individual faucets. It will be shocking.

Yes I have the WH in single best location . Might be able to run 1 dedicated line to kitchen this is the main problem area.

 

[TKB4]

Or for a budget-friendly solution, just drill out the restrictors of those 2 gpm faucets.
Even if you have to buy water, when the price is $0.01/gallon, running the tap is cheaper than those silly recirculation systems.
If you feel bad about it, catch the water in a bucket and use it to water plants or flush the toilet.

I attempted to explain cost/risk etc. to my 98 year old mother who insisted on carrying a container of recycled rinse water up the stairs (therefore not holding the handrail).
"One gallon of water costs $0.01, the last time you fell and hit your head the bill was $50,000; just use the faucet and buy the water. You have enough money."
But no, her skull is too thick. She insists on keeping her old habits.

Sounds like my mother. Lived in depression and can't waste anything . I am on well so no significant cost for the water. Might try removing restrictor first The wait is over 1 minute at the kitchen now. The good news is the master shower only takes about 15 seconds.

 

[Jackfre]

Yes, run the line to the kitchen. Do before and after tests an let us know how it goes. Plumbing codes will not allow 3/8", but the codes on pipe sizing have also not followed the flow restricted faucet/showerhead regulations. In the last drought, which actually continues here, my four neighbors surrounding my place lost their wells and had to add storage tanks. I have a 5'diameterx 75' deep 100 yr old dug well so the storage is in the well. Water conservation is more important energy conservation here. But, they do seem to ge hand in hand.

 

[ncowan]

Hello.
I live in a (very) small condo here in San Diego, and being alone my hot-water needs are pretty minimal, but when I needed a new heater a few years ago I looked at tankless. Not sure why I didn’t purchase then but I ended up getting the smallest tank heater I could, figuring it would be enough, but if I want anything longer than an 11-minute HOT shower I am out of luck. So now I am re-considering getting tankless. For a small place like mine, the price is very reasonable, less than $1,000 (my tank was about $350). If it is a worthwhile endeavor I am willing to pay the added expense but have heard good things... and bad things... about tankless water heaters. Can you tell me yours?

 

[ncowan]

Hello.
I live in a (very) small condo here in San Diego, and being alone my hot-water needs are pretty minimal, but when I needed a new heater a few years ago I looked at tankless. Not sure why I didn’t purchase then but I ended up getting the smallest tank heater I could, figuring it would be enough, but if I want anything longer than an 11-minute HOT shower I am out of luck. So now I am re-considering getting tankless. For a small place like mine, the price is very reasonable, less than $1,000 (my tank was about $350). If it is a worthwhile endeavor I am willing to pay the added expense but have heard good things... and bad things... about tankless water heaters. Can you tell me yours?

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